1. Field of the Invention
The present invention is directed to a method for operating a nuclear magnetic resonance tomography apparatus, and in particular to such a method for acquiring at least two differently weighted images.
2. Description of the Prior Art
A method for the operation of a nuclear magnetic resonance tomography apparatus for the acquisition of at least two differently weighted images which includes the following steps is known from U.S. Pat. No. 5,168,226. An examination subject is brought into a constant, uniform magnetic field. In an excitation phase, nuclear spins in the examination subject are caused to precess by activating a first radio-frequency pulse. In a read-out phase, RF-refocusing pulses are multiply emitted in succession, each of these being followed by at least one nuclear magnetic resonance signal that is spatially-encoded by phase-encoding and read-out gradients. The nuclear magnetic resonance signals acquired in the read-out phase are sampled and nuclear magnetic resonance signals that lie closer to the excitation phase are each entered in a row of a k-space of a first raw data matrix and nuclear magnetic resonance signals lying farther from the excitation phase are respectively entered into a row of a k-space of a second raw data matrix. These steps are repeated until all rows of the raw data matrices have been filled. An image is produced from each raw data matrix by Fourier transformation.
The fact is thereby exploited that nuclear magnetic resonance signals that closely follow the excitation are significantly more weakly T2 weighted than nuclear magnetic resonance signals that lie chronologically farther from the excitation. Expressed in other terms, the nuclear magnetic resonance signals following closely after the excitation are weighted with proton density and the nuclear magnetic resonance signals lying farther from the excitation are T2-weighted. A technique referred to as the "shared echo" technique is employed in the method disclosed in U.S. Pat. No. 5,168,226, i.e. only the central rows of each raw data matrix-which essentially determine the contrast-are separately measured for each raw data matrix, whereas the edge rows-which essentially determine the resolution-are measured only once and are used in common for both raw data matrices.
An article "RARE Imaging: A Fast Imaging Method for Clinical MR", Hennig et al., in the periodical "Magnetic Resonance in Medicine", Vol. 3, pp. 823-833 (1986), likewise discloses a turbo-spin echo sequence. It is noted in this article that the amplitudes of an echo train differ due to the T2 relaxation and that, dependent on the classification of the echoes into a raw data matrix, this can lead to pronounced artifacts. In order to avoid this, suitable phase-encoding sequences are proposed in the article. The acquisition of data sets for two images from one pulse train is not discussed.
European Application 571 212 likewise discloses a turbo-spin echo sequence wherein two echoes, respectively employed for producing separate images, are acquired after every radio-frequency refocusing pulse by inversion of the read-out gradient. A different T2 weighting of the two images is thereby achieved by appropriate classification of the echo signals into the two corresponding raw data matrices. The echo signals following closely after the excitation are entered into the middle rows in the first raw data matrix; later echo signals (i.e. echo signals more affected by T2 decay), by contrast, are entered into the second raw data matrix.
After each excitation, an equal number of signals for the raw data matrices are acquired for the two differently weighted images in the known methods.
In many instances, however, nuclear magnetic resonance signals that are already relatively strongly T2 weighted must be used for the proton-density-weighted image. This, however, leads to pronounced edge oscillation artifacts in the proton density image as well as to a mixed contrast that distinctly differs from the proton density image of a conventional spin echo sequence.